This invention relates to plating technology. More specifically, it relates to silicon wafer electroplating technology. Even more specifically, the invention pertains to particular apparatus and methods for controlling plating solution flow dynamics and electric field shape during electroplating.
Electroplating and electroless plating have many applications. One very important developing application is in plating copper onto semiconductor wafers to form conductive copper lines for xe2x80x9cwiringxe2x80x9d individual devices of the integrated circuit. Often this plating process serves as a step in the damascene fabrication procedure.
A continuing issue in modem VLSI wafer electroplate processing is quality of the deposited metal film. Given that metal line widths reach into the deep sub-micron range and given that the damascene trenches often have very high aspect ratios, electroplated films must be exceedingly homogeneous (chemically and physically). They must have uniform thickness over the face of a wafer and must have consistent quality across numerous batches.
Some wafer processing apparatuses are designed to provide the necessary uniformity. One example is the SABRE(trademark) clamshell electroplating apparatus available from Novellus Systems, Inc. of San Jose, Calif. and described in U.S. Pat. Nos. 6,156,167 and 6,139,712, which are herein incorporated by reference in their entireties. The clamshell apparatus provides many advantages for wafer throughput and uniformity; most notably, wafer back-side protection from contamination during electroplating, wafer rotation during the electroplating process, and a relatively small footprint for wafer delivery to the electroplating bath (vertical immersion path).
Modifications to the xe2x80x9cclamshellxe2x80x9d and its associated plating environment for improved wafer uniformity and quality have been described in U.S. Pat. Nos. 6,074,544, 6,110,346, 6,162,344, and 6,159,354 which are herein incorporated by reference in their entirety. The described modifications relate to methods for using variable currents, improved mass transfer, and electric potential shaping.
Although plating uniformity has improved, as mentioned, there is a continuing need for higher uniformity and plating quality. This is especially true with regard to what a wafer xe2x80x9cseesxe2x80x9d during plating, that is, the local environment of the wafer during plating. Plating solution flow patterns and velocity, bubbles, electric field shape, and the like affect the quality of the deposited metal film. One issue of particular importance is the electrolyte velocity distribution across the wafer surface. For many combinations of apparatus and operating conditions, the electrolyte velocity varies significantly in the radial direction across the wafer surface. Because the plating rate and quality is a function of local velocity, this condition causes uneven plating thickness and/or quality.
With the deleterious effects of bubbles as well as specific flow patterns and electric field shape on the quality in mind, tighter control of certain design parameters should be realized. What is needed therefore is improved technology for controlling plating solution flow dynamics and electric field shape during electroplating.
The present invention provides apparatus and methods for engaging a work piece during plating and plating a material onto the work piece. Although specific embodiments of this invention are described in terms of electroplating, the invention is also embodied in apparatus for electroless plating (plating metal onto a work piece via non-electrolytic reduction of metal ions from plating fluids). Generally the invention controls plating solution fluid dynamics and electric field shape (in the case of electroplating). It keeps the wafer""s local plating environment relatively uniform and bubble free to deposit a film of consistent thickness and quality across the wafer surface. The invention employs an apparatus for holding the work piece in a manner that facilitates electrolyte circulation patterns in which the electrolyte flows from the center of the work piece plating surface, outward toward the edge of the edge of the work piece. The apparatus holds the work piece near the work piece edges and provides a flow path for electrolyte to flow outward away from the edges of the work piece plating surface. That flow path has a xe2x80x9csnorkelxe2x80x9d shape in which the outlet is higher than the inlet. In addition, the flow path preferably a slot shape that spans much or all of the circumference of holding apparatus. It may be made from a material that resists deformation such as certain ceramics.
In one aspect, the invention provides an apparatus for engaging a work piece during plating. The apparatus may be characterized by the following elements: (a) a cup having a circumferential side wall defining an interior region and a lip within the interior region; (b) a field shaping element; (c) a flow path defining a passage for plating fluid to flow from inside the apparatus to outside the apparatus; and (d) a cone having a work piece contact surface that fits within the cup""s interior and can contact the work piece in a manner that holds the work piece in a fixed position against the cup""s lip. In this embodiment, the cup is arranged such that lip can support the work piece while the work piece remains within the interior region. The field shaping element is shaped and sized to affect an electric field shape impinging on the work piece during plating. The field shaping element is also designed for connection with the cup. The flow path resides in a region between the field shaping region and the cup. It has an inlet on the inside of the apparatus and an outlet on the outside of the apparatus and is positioned such that the outlet is at a higher elevation than the inlet when the apparatus oriented for use with the cup above the field shaping element.
The apparatus may include one or more actuators for moving the work piece into and out of the plating fluid, while the work piece is held in position by the cup and cone. These actuators can pivot the work piece about an axis defined on or proximate the work piece.
In a particular embodiment, the cup""s lip is sized and shaped to support a semiconductor wafer work piece. In other words, the apparatus is designed to plate semiconductor wafers (with copper in a specific embodiment). The may include a lipseal made from a material that provides a fluid tight seal with the semiconductor wafer when the wafer is held in place by the cone. This seal protects the backside of the wafer from the plating environment. As an example, the lipseal material may be an elastomer.
The cup can be made from various materials. Preferably, the material resists plating chemicals, resists deformation when pressure is applied from the cone (it is mechanically rigid), and possesses mechanical strength. Among the suitable materials are certain ceramics (alumina, zirconia), plastics, fluoropolymer or other plastic-coated metals and/or ceramics, glasses or glass-coated metals, and composite materials. Note that in many embodiments, the portion of the cup facing the inside of the apparatus is most prone to deformation. Therefore, sometimes only that portion of the cup need be made from special material.
The cup and the field shaping element can form a unitary element of the apparatus or they can be separate elements held in fixed positions with respect to one another by a fastener. In the latter case, the fastener allows a separation distance between the cup and field shaping element to be adjusted to thereby adjust a dimension of the flow path.
The flow path may have a slot shape that is substantially coextensive with the cup""s circumferential side wall. The slot may be continuous around the entire perimeter or it may be discontinuous. In an alternative embodiment, the flow path may have a hole shape. In this embodiment, the apparatus will generally include a plurality of hole shaped flow paths distributed about the cup""s circumferential sidewall. Either flow path can have a xe2x80x9csnorkelxe2x80x9d type design, wherein the inlet is lower in elevation than the outlet. Particular shapes and designs, not meant to limit the scope of the invention will be described below.
In another aspect, the invention provides a method of plating a material onto a work piece. Such method may be characterized by the following sequence: (a) holding the work piece between a cone and a cup of an apparatus; and (b) while plating, directing plating fluid through a flow path defining a passage for plating fluid to flow from inside the apparatus to outside the apparatus. The flow path should have an inlet and an outlet in the snorkel arrangement as described above. In this manner, any gas present in the flow path travels toward the outlet due to its buoyancy and entrainment in the fluid flow stream. In this method, the cup and cone are preferably designed as described above.
In one embodiment, the plating fluid is directed toward a substantially flat plating surface of the work piece (e.g., a wafer""s active surface), thereby setting up a circulation pattern in which the plating fluid flows along the plating surface and through the flow path. When the work piece has a substantially flat plating surface, immersion of the work piece can take place at an angle in which the plating surface is not parallel to a plane defined by the plating fluid surface. In this arrangement, buoyancy aids in the elimination of gas from the work piece plating surface. The work piece can also be rotated about an axis substantially parallel to the circumferential side-wall during immersion and plating.
These and other features and advantages of the present invention will be described in more detail below with reference to the associated figures.